1. Field of the Invention
The present invention relates to a semiconductor device, in particular, to a semiconductor device having a guard ring.
2. Description of the Background Art
Generally, a power semiconductor requires a high main breakdown voltage retaining capability. For such retention of breakdown voltage, guard ring structures are generally utilized. Basic structures and applied structures thereof are disclosed in, for example, the following document or the like:    B. Jayant Baliga, “Power Semiconductor Devices”, the United States, PWS PUBLISHING COMPANY, pp. 98-103
A guard ring structure is provided with an impurity region having a floating potential and surrounding an emitter to alleviate surface electric field and retain breakdown voltage.
If guard rings are provided up to the end of a semiconductor substrate, an electric field peak comes in the semiconductor substrate in the vicinity of its central portion in which an element formation region is provided. Hence, the guard rings are not formed up to the end of the semiconductor substrate.
Intervals between the guard rings are adjusted so that an electric field peak is maximal just below a guard ring electrode attached to an outermost guard ring. Unfortunately, this limits improvement of breakdown voltage just below the guard ring electrode attached to the outermost guard ring.
Also, the guard ring structure unfortunately requires a large area to retain breakdown voltage.